This invention relates to a medical apparatus for enhancing and for correcting skeletal mechanics. More specifically, this invention relates to the correction of certain bone alignment deformities that impair optimal biped mechanics.
Excessive pronation (hyperpronation) is caused by abnormal motion between two bones of the foot; the ankle bone (talus) and the heel bone (calcaneus). This abnormal motion will eventually lead to anatomical mal-alignment both proximally and distally. The abnormal motion is due to obliteration or closure of a naturally occurring space (sinus) formed between the talus and calcaneus.
This sinus is referred to anatomically as the sinus tarsi. In anatomical terms, the sinus tarsi is located anterior to the subtalar joint and posterior to the talocalcaneo-navicular joint. As will be described in greater detail below, the subtalar joint is formed by the posterior talar facet of the calcaneus and the posterior calcaneal facet of the talus. The talocalcaneonavicular joint is formed by the middle and anterior calcaneal facet of the talus and middle and anterior talar facet of the calcaneus.
Generally speaking, when a human biped is walking or running, the individual's talus acts as a “torque converter” to transfer the weight of the body to the foot. This weight transfer is accomplished via the motion of the subtalar joint, which is mainly movement of the talus onto the calcaneus. The normal mechanics of the subtalar joint produces a triplanar motion-motion through all three anatomical planes. This motion consists of supination, and pronation. Pronation occurs when the talus moves medially (inward), anterior (forward) and plantarly (inferiorly). Supination occurs when the talus moves laterally (outward), posteriorly (backward) and dorsally (upward). Normally, there should be approximately a two-to-one ratio of supination to pronation.
Some individuals suffer as a result of abnormal motion of the subtalar joint. This is often referred to as excessive pronation or, more specifically, hyperpronation. The pathomechanics of hyperpronation leads to significant deleterious effects to the bony architecture of the talus and calcaneus both proximally and distally. Hyperpronation is defined by excessive talar deviation medially (inward), anteriorly (forward), and plantarly (inferiorly).
Hyperpronation is detected and diagnosed through physical examination of the foot, both non-weight bearing and weight bearing examination, as well as radiographic evaluation of the foot. Non-weight bearing examination of hyperpronation is achieved by applying pressure to the fifth metatarsal head region of the foot to dorsiflex the foot (push the foot toward the front of the shin) and if the foot turns out-ward hyperpronation is present. In the weight-bearing examination, the person stands on his/her feet and the examiner observes both pronation and supination of the subtalar joint. Normally the foot should be in a “neutral” position, that is, neither pronated nor supinated. If the foot is in a pronated position while full weight is on the foot, the foot is considered hyperpronated.
Radiographic evaluation of hyperpronation is seen by examining the weight-bearing anterior-posterior (top to bottom) view and the lateral (side) view. These two projections show the relationship of the talus to the other foot bones. If the talus is medially (inward) and/or anteriorly (forward) deviated and/or plantarflexed (inferiorly) displaced hyperpronation is present.
Previous implants have been designed for insertion into the sinus tarsi in an attempt to treat foot disorders. In this, one envisioned design included a mushroom-shaped implant with a stem protruding from the bottom. The implant was held in place by inserting the stem into a hole drilled into the dorsum of the calcaneus. Unfortunately, drilling can weaken the calcaneus and often resulted in direct or ultimate fracture. Moreover, the stem of the implant is subject to fracture which, of course, again leads to failure of the procedure. Also, the surgical procedures necessary for implantation is somewhat and subject to physician error.
In another previously known design, an implant is threaded on an outer surface and screwed into the sinus tarsi. This implant is usually composed of high molecular weight polyethylene. Unfortunately, this device can only be gas sterilized. This allowed the device to deform under the compressive pressure to which it is subjected under normal post-operative condition. Furthermore, it was difficult to accurately locate the device properly within the sinus tarsi.
In yet another design, a cylindrical implant made of a titanium alloy is threaded on an outer surface. However, this implant only corrects one portion of the deformity while both the lateral and medial portions of the sinus tarsi need correction. Furthermore, a titanium implant is much harder than surrounding bone matter. This can lead to bone wear and/or deformation. In addition, fluoroscopy is required in order to verify the position which exposes a patient to radiation. The procedure for insertion requires two separate incisions on the medial and lateral aspect of the foot and calls for a below-the-knee cast for two weeks post-operatively. Finally, the implant is made available in a series of sizes. These implants vary in size, one from the next, by specific increments. Gaps in this series can lead to under and over correction.
The problems suggested in the preceding are not intended to be exhaustive but rather are among many which may tend to reduce the effectiveness of sinus tarsi implants known in the past. Other noteworthy problems may also exist; however, those presented above should be sufficient to demonstrate that previously known sinus tarsi implants will admit to worthwhile improvement.